This invention broadly relates to an electrostatic coating composition comprising a corrosion resistant metal particulate component comprising aluminum-containing metal particulates having a phosphate and/or silica-containing insulating layer; and a glass-forming binder component. This invention also broadly relates to a method for using this coating composition to electrostatically coat the metal substrate of an article.
A number of corrosion resistant coatings have been suggested for protecting the metal substrate of gas turbine components, as well as other articles requiring corrosion protection. Some of these corrosion resistant coatings are applied to or deposited on the metal substrate as sacrificial coatings. These coatings are “sacrificial” in that they preferentially react with and are used up by the corrodants, thus protecting the underlying metal substrate from attack by these corrodants.
Sacrificial corrosion resistant coatings for metal substrates can be formed from chromium or more typically aluminum, or from the respective oxides (i.e., alumina or chromia), by diffusion processes or techniques such as chemical vapor deposition or pack cementation. See, for example, commonly assigned U.S. Pat. No. 5,368,888 (Rigney), issued Nov. 29, 1994 (aluminide diffusion coating); commonly assigned U.S. Pat. No. 6,283,715 (Nagaraj et al), issued Sep. 4, 2001 (chromium diffusion coating); commonly assigned U.S. Patent Application 2004/0013802 A1 (Ackerman et al), published Jan. 22, 2004 (metal-organic chemical vapor deposition of aluminum, silicon, tantalum, titanium or chromium oxide on turbine disks and seal elements to provide a protective coating). Diffusion coating processes and techniques such chemical vapor deposition can be complicated and expensive processes for applying sacrificial corrosion resistant coatings to the metal substrate. These diffusion coatings also require some of the metal in the underlying substrate to be able to diffuse therefrom to form the coating.
These sacrificial coatings can also be applied to the metal substrate as aqueous compositions comprising phosphate binder systems and aluminum/alumina particles. See, for example, U.S. Pat. No. 4,606,967 (Mosser), issued Aug. 19, 1986 (spheroidal aluminum particles); U.S. Pat. No. 4,544,408 (Mosser et al), issued Oct. 1, 1985 (dispersible hydrated alumina particles). The phosphate-containing binder systems of these aqueous coating compositions typically comprise other binder materials, including chromates. See, for example, U.S. Pat. No. 3,248,249 (Collins, Jr.), issued Apr. 26, 1966; U.S. Pat. No. 3,248,251 (Allen), issued Apr. 26, 1966; U.S. Pat. No. 4,889,858 (Mosser), issued Dec. 26, 1989; U.S. Pat. No. 4,975,330 (Mosser), issued Dec. 4, 1990.
These aqueous coating compositions comprising phosphate-containing binders are typically applied by standard “wet spray” methods commonly used for spray painting. “Wet spray” methods are relatively easy and uncomplicated methods for applying these aqueous coating compositions to the surface of the metal substrate of the article to be coated. However, due to the lack of precision of “wet spray” methods for applying coatings and to ensure adequate coverage of the surface of the metal substrate to be coated, a significant amount of the aqueous coating composition does not end up on the metal substrate, but is instead lost due to “overspraying” thereof. Because of environmental concerns that can be created by such “overspraying,” aqueous coating compositions comprising phosphate-containing binders that are substantially free of chromates have been developed. See, for example, U.S. Pat. No. 6,368,394 (Hughes et al), issued Apr. 9, 2002 (substantially chromate free phosphate binder component). Even so, the lack of precision of “wet spray” application methods still makes it difficult to adequately and uniformly apply the coating composition to the surface of the metal substrate, especially a vertical extending surface, without significant and wasteful “overspraying” thereof.
Accordingly, there is still a need for sacrificial coating compositions that: (1) provide corrosion resistance for the underlying metal substrate; (2) can be applied to the metal substrate by relatively uncomplicated and inexpensive methods; (3) can be applied so as to adequately and uniformly cover the surface of the metal substrate, especially when that surface is vertical; (4) reduce or minimize wasteful “overspraying;” and (5) do not affect the intrinsic properties of the base metal or have a large interaction zone at the coating-metal interface.